Sounds simply like an error to me. For Future Trunks to have recieved the sword, the box holding Tapion had to have been opened using the Dragonballs.
After the arrival of the androids, when Trunks was still just a baby, Piccolo's killed fighting them. This in turn kills Kami, which makes the Dragonballs obsolete.
Now, one could assume the box was opened before the arrival of the Androids. However, who would have been strong enough to defeat Hildegarn? Tapion? In the movie, it was stated by Gohan and Goku that he had 'Incredible power" but we never really saw him use it.
Then we have the problem of when Whatshisname (Can't remember the name of the Warlock who brought the box to Earth) didn't show up in the film until after the Buu Saga.
In the end, this kind of topic can be argued to death. I'm certainly not saying it's impossible that it's where Trunks got his sword. I'm just saying it's highly, highly unlikely.
Quite possibly the only reason it would be possible would be due to the fact that in a series like Dragonball, sometimes things just aren't explained and you're supposed to accept them

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